CreateQuery t_map_null (children 3)
 Identifier t_map_null
 Columns definition (children 1)
  ExpressionList (children 2)
   ColumnDeclaration a (children 1)
    DataType Map (children 1)
     ExpressionList (children 2)
      DataType String
      DataType String
   ColumnDeclaration b (children 1)
    DataType String
 Storage definition (children 2)
  Function MergeTree (children 1)
   ExpressionList
  Identifier a